Signal processing devices have been suggested by the prior art for providing for the processing and storage of signals by utilizing a piezoelectric substrate capable of propagating acoustic wave signals on a selected surface thereof and a semiconductor substrate positioned adjacent and spaced from such surface. Appropriate techniques are utilized for altering the conductivity pattern in the semiconductor substrate in accordance with the wave form of an acoustic wave signal that is propagated along the selected surface of the piezoelectric substrate so that a representation of the acoustic wave signal is effectively and temporarily stored therein. Such techniques for altering the conductivity pattern include applying a signal uniformly over the interaction region which comprises the regions at or near the surfaces of the substrates and the spatial region therebetween so that a second signal which is propagated along the surface of the piezoelectric material interacts with the uniformly applied signal to alter the conductivity pattern in the semiconductor substrate, the altered conductivity pattern representing the stored propagated signal. A further signal subsequently propagated along the piezoelectric substrate surface thereupon interacts with the stored altered conductivity pattern, the interaction thereby producing an output signal at an electrode of the semiconductor substrate which represents the correlation of the convolution of the two interacting signals depending on the direction of propagation of the further signal along the piezoelectric surface. Certain structural embodiments of such technique have been discussed in the articles of Stern and Williamson, "New Addaptive-Signal-Processing Concept" in Electronic Letters, Vol. 10, No. 5, dated 7 March 1974, and of Bers and Cafarella, "Surface State Memory in Surface Acoustoelectric Correlator", Applied Physics Letters, Vol. 25, No. 3, dated August 1, 1974, and in the copending applications, Ser. No. 555,367 of Stern et al., filed on Mar. 5, 1975, and now U.S. Pat. No. 4,016,412; Ser. No. 672,345 of Stern et al., filed on Mar. 31, 1976; and Ser. No. 672,344 of Stern et al., filed on Mar. 31, 1976.
The major problems with such devices have been that a relatively long time period is required in order to store a signal in the form of such altered stationary conductivity pattern in the previously disclosed embodiments and, once stored, the signal can remain stored therein only for a relatively short time period. In utilizing trap techniques, as disclosed in the above Bers et al. article and the patent applications, for example, the time required to store a signal may be in the order of 0.1 to 1 microsecond (.mu. sec.), while the signal can be held in storage only for about 0.1 to 1 millisecond (msec.), or less. The usefulness of such devices thereby becomes limited because of the relatively long storing or "write" time period and the relatively short storage time period.
Another embodiment of such technique for improving the write and storage time periods of these devices is disclosed in the article of Ingebrigtsen et al., "A Schottky-Diode Acoustic Memory and Correlator," Applied Physics Letters, Vol. 26, No. 11, dated June 1, 1975, and in the copending application Ser. No. 690,601 of Stern et al., filed May 27, 1976. In a preferred embodiment discussed in such article and application an array of diodes, such as Schottky diodes, are formed in appropriate holes in a thermally grown silicon dioxide layer which is present on a selected surface of a substrate of semiconductor material, such as n-type silicon, and an island, or overlay, comprising a conductive material, for example, a metal such as gold, is formed over each of the diodes of the array to increase the capacitances thereof. An interaction region thereby exists which region includes the regions at or near the adjacent surfaces of the substrates and the spatial region therebetween. A signal applied uniformly over the interaction region to produce time varying properties thereof provides a substantially uniform charge on each of the diodes.
A surface wave signal which is propagated along the adjacent surface of the piezoelectric substrate thereby interacts with the uniformly applied signal so as to alter the charge placed on the Schottky diodes, which interaction induces further charges proportional to the amplitude of the propagated signal on the diodes so as to alter the uniform charge on the array and thereby to provide an altered stationary conductivity pattern in the semiconductor substrate which represents the wave pattern of the propagated signal.
If a further signal is subsequently propagated along the surface of the piezoelectric substrate, it interacts with the altered stationary conductivity pattern representing the stored signal and provides an output signal at an appropriate electrode of the semiconductor substrate, which output signal represents either the correlation or the convolution of the stored signal with the further signal depending on which direction the further signal is propagated.
The use of Schottky diodes in such configuration provides forward charging or write times as short as 10.sup.-9 seconds and reverse bias "storage" times of about 0.1 seconds, as compared to the structures suggested prior thereto which produced write times of as long as 0.1 to 1 microseconds and storage times of only 0.1 to 1 milliseconds. Despite such improvement, however, it is desirable to provide even better performance, particularly in producing devices having even longer storage times at least an order of magnitude longer, e.g., as high as 1.0 second or more.